Method for the partial fusion of objects

ABSTRACT

A method for locally heating objects, in particular thin sheet metal, by charging the objects by means of a plasma ignited between two electrodes. In order to keep the thermal stress of a subject as low as possible outside of the zone to be heated it is provided that the machining such as spot welding or burning through a breakthrough occurs with merely one plasma pulse which is produced by applying a voltage pulse exceeding the arc-over voltage of the gap between the electrodes.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of U.S. patent application Ser. No. 09/294,612, filedApr. 19, 1999, now U.S. Pat. No. 6,215,588

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the partial fusion of objects.

2. Description of the Prior Art

In known such methods a substantially continuously flowing plasma isused, mostly for hardening the surface of objects made of steel.

A laser beam or an electron beam is mostly used for other methods, e.g.for welding, in particular for spot welding thin sheets, or forproducing a breakthrough in thinner metallic objects. This leads to thedisadvantage, however, that laser welding processes require a verylaborious preparation of the parts to be welded, which must be joinedwith a very high precision in order to enable their welding by means ofa laser beam. The same also applies with respect to methods usingelectron beams. Moreover, the equipment required for performing suchmethods is very complex in a constructional respect.

SUMMARY OF THE INVENTION

It is the object of the present invention to avoid such disadvantagesand to provide a method of the aforementioned kind which allows a simplemachining of objects, in particular the production of spot welds or theburning of breakthroughs.

This is achieved in accordance with the invention by machining withmerely on plasma pulse, which is produced by applying a voltage pulseexceeding the arc-over voltage of the gap between the electrodes.

As a result of the proposed measures it is possible with relativelyroughly prepared parts to join the same by means of spot welding.Measures will substantially suffice as are also required in electricresistance spot welding.

A very high ejection speed of the plasma pulse is secured by theignition of the arc by exceeding the arc-over voltage of theanode-to-cathode gap, so that this pulse will impinge upon the parts tobe welded with a high kinetic energy. The plasma pulses thus producedwill reach very high temperatures of 20,000 to 50,000° C. and will causeadequate fusion of the mutually adhering surface areas of the parts tobe joined despite a short action period of e.g. 10⁻⁵ to 10⁻⁰ seconds andwill thus cause a secure connection.

Machining in a protective gas atmosphere helps avoid the formation ofoxide layers on the subjects, with the gas used for the production ofthe plasma, mostly argon or helium, appropriately being simultaneouslyused as inert gas.

If the plasma pulse has a duration of about 10⁻⁵ to 10⁻⁰ seconds,preferably 10⁻⁴ to 10⁻¹ seconds, relatively compact devices or plasmatorches may be used which can be operated at a relatively high outputover a short period.

For a weld seam from a number of welding spots, the object to be joinedare charged with a number os successive plasma pulses while the objectsare moved relative to the electrodes and the electrodes are kept at aconstant distance from the objects, a repetition frequency of the plasmapulses of 5 to 100 Hz being provided.

In such a device it is possible in a simple way to charge the subject(s)to be machined with a sequence of very short plasma pulses. In thecourse of charging the capacitor battery the arc-over voltage of theanode-to-cathode gap will be exceeded and thus an arc will be formedthrough which there will be a discharge of the capacitor battery. Thearc will extinguish as soon as the voltage of the capacitor batterydrops below the arc drop voltage. As a result of a respectivedimensioning of the charging circuit and the discharge circuit of thecapacitor battery with respect to the time constants it is possible todetermine both the arc duration in each cycle as well as the repetitionfrequency. The arc which thus burns only very briefly produces plasmapulses which, as a result of the very rapid heating of the ambient gas,exit with a very high speed from the outlet opening of the chamber ofthe plasma torch and impinge upon the objects to be joined or the objectto be provided with a breakthrough and as a result of their hightemperatures ensure the fusion or the melt-through of the object(s).

Short pulse durations of the plasma pulses of 10⁻⁵ to 10⁻⁰ seconds forexample and a repetition frequency of 7 to 100 Hz are required for thecareful treatment of the objects to be machined. As a result of theseshort operating times of the individual plasma pulses, the thermalstress on the objects is kept low and thus the danger of distorting themostly very thin or thin-walled objects is substantially avoided.

Initiating even before reaching the arc-over voltage of theanode-to-cathode gap allows keeping the arc duration, and thus theplasma pulses, extremely short without having to make any particularlygreat efforts concerning a particularly low-resistance arrangement ofthe discharge circuit of the capacitor battery.

It is principally also possible to also use a technical AC network or avoltage source supplying a high-frequency AC current in conjunction witha phase controller instead of the capacitor battery as a voltage supplyfor the plasma torch. In this respect it must be ensured in the case ofelectrodes made of different materials that merely equally polarisedhalf-waves are partly connected through so that voltage pulses with thesame polarity are always applied to the different electrodes andsubstantially the same ratios as in the supply of the plasma torch withDC voltage pulses, like from a capacitor battery for example, areobtained.

In cases in that both are electrodes made from the same material, pulseswith different polarity can be applied to each of the two electrodes.

As electrodes which are made of different materials for the purpose ofachieving a longer service life are usually charged with the samepolarity in plasma torches, the terms “anode” and “cathode” aregenerally used in the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in closer detail by reference to theenclosed drawing, wherein:

FIG. 1 shows a sectional view through a device with a plasma torch inaccordance with the invention;

FIG. 2 shows a top view on the holder plus a plasma torch in accordancewith FIG. 1;

FIG. 3 shows a sectional view through the plasma torch in accordancewith FIGS. 1 and 2 on an enlarged scale;

FIG. 4 shows a sectional view through a coolant chamber of the anodecontact part;

FIG. 5 shows a sectional view through the centering sleeve;

FIG. 6 shows a first embodiment of a voltage supply for a plasma torchand

FIG. 7 shows a further embodiment of a voltage supply for a plasmatorch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A holder 1′ is provided in the embodiment in accordance with FIG. 1,which holder is provided with bores 4′ for receiving contact pins 9′,with the contact pins 9′ being axially bored through. The contact pins9′ are provided with an outside thread 29 in a zone outside of theholder 1′ on which terminal nuts 30 are screwed and between which cablelugs 31 of connecting lines 6 (FIG. 2) are clamped.

The rear end of the contact pins 9′ is arranged for the connection oftubes through which cooling water can be supplied.

Furthermore, a gas supply line 3′ is held in the holder 1′ which—as canbe seen from FIG. 2—is connected with a gas tube 36 through a radialduct 32 which is outwardly occluded with a grub screw 33 and through anaxial bore 34 which opens into the same and into which a hose nozzle 35is screwed. A gas required for producing the plasma can be suppliedthrough said gas tube.

The gas supply line 3′ is provided in the zone of the radial duct 32with slots 37 through which the gas can flow into the interior of thegas supply line 3′. The gas supply line 3′ is secured in its position bymeans of the screw 39 which engages in the same.

As can be seen from FIG. 1, the contact pins 9′ project in theirspring-loaded idle position beyond the face surface 38 of the holder 1′and engage in the jacket surface of a plasma producer 11′ which isarranged as a module. The same also applies for the gas supply line 3′which, when the plasma producer 11′ is mounted, engages in the same.

The plasma producer 11′ which is arranged as a module is held by meansof a pipe bracket 40 whose rigid part held on the face side 38 of theholder 1′ is held with pins 42. Pipe bracket 40 is provided with a joint43 whose axis extends perpendicularly to the axis of holder 1′.

The holding part 18′ of cathode 19′ is formed by a collect chuck inplasma producer 11′, which chuck is made from an electricallywell-conducting material. Said collect chuck is held in the usual mannerin a receiver 44 which is screwed into a contact part 45.

Said contact part 45 is provided with a coolant chamber 46 which isconnected with a connecting opening 48 through a radial duct 47. Saidconnecting opening 48 is in true alignment with the contact pins 9′ whenholder 1′ is mounted in plasma producer 11′.

An adjusting nut 49 is provided for tensioning and loosening the collectchuck 18′, which adjusting nut rests on the upper face surface ofreceiver 44 through two seals 50, as a result of which any escape ofcoolant liquid is prevented. Receiver 44 is also supported on thecontact part 45 through a seal 51 for sealing the coolant chamber 46.

An O-ring 52 is provided for further sealing the coolant chamber of thecontact part 45, which O-ring is inserted into a groove of a bore 53which is penetrated by receiver 44.

In order to secure the axial setting of the cathode 19′ during thetensioning of the collect chuck 18′, adjusting nut 49 is provided with athreaded through-bore 90 into which a stop 91 is screwed which engagesinto the collect chuck 18′. Said stop 91 is provided with a smooth head94 in which a circular groove is incorporated for receiving an O-ring 95which is used for sealing the interior of the collect chuck 18′.

A counternut 92 is provided to secure the position of stop 91 which isadjustable by means of screwdriver which is inserted into the face-sidedslot 93. Counternut 92 simultaneously ensures a torsionally rigidconnection between the stop 91, on which rests cathode 19′, and theadjusting nut 49.

Stop 91 ensures that during the tensioning of the collect chuck cathode19′ can no longer be axially moved with respect to anode 15′ by collectchuck 18′, because the adjusting nut 49 rests on the face side ofcontact part 45 and anode 15′ is fixed with respect to the same.

Contact part 45, which is used for making the contact of cathode 19′,rests on an intermediate part 55 by interposing a seal 54, whichintermediate part is made from an electrically insulating material suchas ceramic. Said intermediate part 55 determines the chamber 27′ whichis connected with a connecting opening 57 through a radial duct 56.

The radial ducts 47 and 56 are provided with circular grooves 58 inwhich O-rings 59 are arranged. They are used for sealing the contactpins 9′, which engage in these ducts, and the gas supply line 3′.

A distributor ring 59′ is arranged in chamber 27′ which is provided withbores 60 which are arranged distributed over the circumference and whosediameters in both directions of rotation increase with an increasingangle towards the radial duct 56. The axial bore of the distributor ring59′ is penetrated by the cathode 19′. An annular space 61 remainsbetween the inner wall of the intermediate part 55 and the distributorring 59′.

The intermediate part 55 rests on the anode contact part 63 supportedthrough a seal 62. A clamping sleeve 64 is screwed into an inner thread65 in said anode contact part 63, with a sealing 66 being interposedbetween the anode contact part 63 and the face surface of the clampingsleeve 64.

The clamping sleeve 64 is provided in the zone of its one end with aconical bearing surface 67 on which rests a diametrically opposedconical jacket surface 68 of a head 69 of an anode 15′ which, like theclamping sleeve 64 and the anode contact part 63, is made of anelectrically well-conducting material.

Anode 15′ is supported with its end averted from head 69 with a furtherhead 70, which by interposing a seal 71 rests on a shoulder of the anodecontact part 63. Anode 15′ penetrates a coolant chamber 46 of the anodecontact part 63.

Anode 15′ is bored through in the axial direction, with a sleeve 73 madefrom an electrically insulating material such as ceramic is disposed onbore 72 and is penetrated by cathode 19′.

Moreover, a centering sleeve 74 is inserted in bore 72 in the zone closeto the orifice of anode 15′, which sleeve is illustrated in closerdetail in FIG. 5 and whose guide surfaces 75 provided on guide ribs 89rest on the jacket surface of cathode 19′.

As is shown in FIG. 4, anode 15′ is provided with radially projectingguide ribs 76 which extend from the anode 15′ having a hexagonal crosssection up to the inner wall of the clamping sleeve 64 and standperpendicular to the axis of the radial duct 47. Guide ribs 76 extendaway from head 70 against the head 69 of anode 15′, with a flow gap 77remaining between the head 69 and the guide ribs 76.

In this way the coolant chamber 46, which is limited on its part by theanode contact part 63 and the clamping sleeve 64, is subdivided by theguide ribs 76.

The two coolant chambers 46 of the contact part 45 and the anode contactpart 63 are mutually connected through a transfer duct.

Which is substantially composed of the axial bores 79 in the contactpart 45 and the anode contact part 63, respectively, and radial bores 80which are coaxial to the radial ducts 47 and open into the axial bores79 bore 78 in intermediate part 55.

Seals 82 are provided in the zone of the bore 81 of the intermediatepart 55.

An insert 83 is provided in the orifice zone of anode 15′, which insertis made of a wear-resisting materials such as a tungsten-cerium oxidealloy and delimits a nozzle aperture 16. The section of anode 15′projecting from the clamping sleeve 64 is encompassed by a ring 96 whichis made of a wear-resistant material and projects axially beyond thenozzle aperture 16 of anode 15′ and defines a pre-chamber 97.

The two contact parts 45 and 63 are encompassed by rings 84 made of anelectrically insulating material and rest on collars 85.

As can be seen from FIG. 1, the pipe bracket 40 is provided in the zoneof the collars 85 of the contact parts 45 and 63 with recesses 86, thuspreventing a short between the two contact parts 45 and 63.

Cathode 19′ is arranged conically at its two ends.

The two contact parts 45 and 63 and the intermediate part 55 aremutually connected by means of the screws 87 shown in FIG. 2 andrepresent the connecting parts which thus ensure a modular arrangementof the plasma producer 11′.

As soon as cathode 19′ is worn off, the plasma producer 11′, which isarranged as a module, can be removed by loosening the straining screw 88and by opening the pipe bracket 40, whereupon the adjusting nut 49 canbe loosened and the cathode 19′ can be removed from the collect chuck.Thereafter the cathode can either be turned round or its conical endscan be re-ground. Then the cathode can be adjusted by means of a caliberwith respect to anode 15′. Then the stop 91 is adjusted while thecollect chuck 18′ is opened and the cathode 19′ is fixed again in thecollect chuck 18′ by means of adjusting nut 49, whereupon module 11′ canbe mounted again.

During the operation a gas such as argon, helium, nitrogen or the likeis blown into chamber 27′ and an arc between the cathode 19′ and theanode 15′ is ignited through a voltage pulse which after a brief periodof time drops below the arc drop voltage, so that the arc goes out. Theplasma pulse thus formed exits through the nozzle aperture 16, passesthrough pre-chamber 97 and impinges upon the subject(s) to be machined.They are fused by the action of the plasma pulse, thus melting abreakthrough or fusing two subjects to be welded, depending on theenergy of the plasma pulse. In the latter case there will be a secureconnection of the two parts during the following solidification afterthe plasma pulse has gone out. In this process these parts aresufficiently pressed together by the kinetic energy of the plasma pulseexiting with a high speed, whereby speeds of 2000 m per second areachieved, thus ensuring a secure connection.

The pre-chamber 97 allows in a very simple way charging the subjects tobe machined with plasma pulses under a protective gas atmosphere. Forthis purpose it is merely necessary to supply the plasma torch 11′ witha substantially constant flow of plasma gas such as argon, helium ornitrogen. Nitrogen can only be used if the subject to be machined iscompatible with a nitrogen atmosphere in the fused stated.

Furthermore, the plasma torch 11′ can be placed on the subject to bemachined with the face side of ring 96 during the production ofindividual welding spots, thus simultaneously defining the distancebetween the electrodes 15′, 19′ and the upper side of the subject.

For special applications such as the production of breakthroughs withvery small diameters it is possible to provide nozzles 16 with verysmall diameters, as small as 10 μm for example. As in such plasmatorches 11′ it is possible to reduce the output appropriately, one canomit cooling ducts in such plasma torches.

FIG. 6 shows a voltage supply for a plasma torch 11′ in accordance withFIGS. 1 to 5, with the voltage supply being provided for the productionof a pulse plasma.

A capacitor battery 130 is connected by way of a charging resistor 131with the connections X1 of a controllable DC voltage source 132. Thecapacitor battery 130 is provided with a fixedly connected capacitor 1C1and a capacitor 1C2 which is connectable parallel to the same through aswitch 1S1. Groups of capacitors can be concerned in both cases.

Said capacitor battery 130 is connected through connecting lines 133,134 with the cathode and anode of plasma torch 22 (not illustrated inFIG. 6).

An RC module is switched in parallel to the capacitor battery 130 whichis formed by a capacitor 1C3 and a resistor 1R1. This RC module forms anHF block circuit in conjunction with a choke 1L1 switched in theconnecting line 134, which choke is provided for the protection of thecapacitor battery 130 against HF signals.

The outputs of an ignition set 135 are further connected to theconnecting lines 133, 134. Said ignition set 135 is connected on theinput side with an AC voltage source X2 and provided with a triggerswitch 1S2 by which an ignition pulse can be initiated when actuated.

During operation, the capacitor battery 130 is charged according to theset voltage of the DC voltage source 132 which is adjustable between 50Vand 300V and the time constant which is co-determined by the capacity ofthe capacitor battery 130 and the line resistances and the chargingresistance.

Once the capacitor battery 130 reaches a voltage which corresponds tothe arc-over voltage of the anode-to-cathode gap 15′, 19′ of the plasmatorch 11′, an ignition of an arc between anode 15′ and cathode 19′ (FIG.1, FIG. 3) and thus the formation of plasma in the orifice zone of theanode 15′ of the plasma torch 11′ will occur.

At the same time the capacitor battery 130 will discharge according tothe time constant given by its capacity, the line resistances and theresistance of the arc. If as a result of this discharge the voltage ofthe capacitor battery 130 drops below the arc drop voltage, the samegoes out and the capacitor battery 130 charges up again, as a result ofwhich the described process is repeated and a frequency is obtainedwhich is determined by the charging and discharging time constants. Theoperation of the ignition set is not required.

For certain applications it can be desirable to determine the ignitiontime of the arc precisely or to initiate such a one prior to reachingthe arc-over voltage of the anode-to-cathode gap 15′, 19′ in order toenable the production of particularly short plasma pulses.

In this case an ignition pulse is initiated by actuating the triggerswitch 1S2 which leads to the ignition of an arc between the anode 15′and the cathode 19′ of the plasma torch 11′ without the capacitorbattery 130 having reached a voltage corresponding to the arc-overvoltage of this gap. In this way the pulse-duty factor, which can beselected between 1:10 and 1:100 and even beyond this figure, can bechanged respectively and the ratio between the arc duration and itspause during a cycle can be changed in the sense of an extension of thearc pause, since the energy of the high-frequency ignition pulses of theignition set 135 is sufficient for igniting the arc, but not formaintaining the same when the voltage of the capacitor battery 130 dropsbelow the arc drop voltage.

The embodiment of the voltage supply for the plasma torch 11′ inaccordance with FIG. 7 is distinguished from the one in accordance withFIG. 6 only in the respect that a mains apparatus 136 is provided inaddition to the capacitor battery 130, which mains apparatus isconnected to an AC voltage network and is provided with a rectifiercircuit. The illustration of the blocking circuit and the choke wasomitted.

The connecting line 133′, which is connected with the negative pole ofthe output of the mains apparatus, is connected to the connecting line133 which is connected to the negative pole of the capacitor battery 130and the connecting line 134′, which is connected to the positive pole ofthe mains apparatus 136, is connected with a subject 138.

An automatic current controller 137 is further connected to mainsapparatus 136.

In operation the mains apparatus 136 will also supply current to plasmatorch 11′ once an arc has been ignited between anode 15′ and cathode19′, with the electric circuit for the mains apparatus being closedthrough cathode 19′ of the plasma torch, the plasma and the subject 138as well as the connecting lines 133′, 133, 134′.

As soon as the arc in the plasma torch 11′ goes out because of the dropof the voltage of the capacitor battery 130 below the arc drop voltage,the electric circuit for the mains apparatus 136 is also interrupted, asits output voltage is not sufficient to maintain an arc between thecathode and the subject 138.

A pulse plasma is also used in a voltage supply pursuant to FIG. 7.

What is claimed is:
 1. A method of joining objects by a weld seam formedfrom a number of welding spots, which comprises the steps of chargingone side of the objects with successive plasma pulses ignited betweentwo electrodes, each plasma pulse being produced by applying a voltagepulse exceeding the arc-over voltage between the electrodes to produceeach welding spot, the electrodes being kept at a constant distance fromthe one side of the objects, moving the objects with respect to theelectrodes and providing a repetition frequency of the plasma pulses of5 to 100 Hz.
 2. The method of claim 1, wherein the objects are chargedin a protective gas atmosphere.
 3. The method of claim 1, wherein eachplasma pulse has a duration of approximately 10−⁵ to 10 seconds.